Method and apparatus for decreasing a transition time of a read head from a write mode to a read mode

ABSTRACT

A device for switching from the write mode to the read mode includes a read head connected between a first and a second input terminal of the differential amplifier, the second input terminal being connected to a constant voltage through a capacitor and to the amplifier output through the resistor. The apparatus includes a circuit for generating a voltage ramp, after a write operation; a circuit for discharging an initial current in the read head proportionally to the voltage ramp between the beginning of the ramp and a first threshold voltage of the ramp; a switch for shorting the resistor to a low value, at least between the first threshold voltage and a second ramp threshold voltage; and a detector for detecting the second threshold voltage and for providing, in the vicinity of the threshold, a progressive control of the switch.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to video tape recorder heads, more particularly to a method and apparatus for rapidly switching a read head to a read mode once a write operation has been carried out.

2. Discussion of the Related Art

Various types of video tape recorders allow for counting a tape's length in real time. To achieve this function, while the tape is being recorded, a rectangular signal, having approximately a 25-Hz frequency, is simultaneously recorded by a control head on a control track of the tape. Thus, when the tape is played back, the rectangular control signal provides a real time count of the length of the tape, independent of the tape speed (fast search, slow motion, normal reading mode . . . ). Additionally, some video tape recorders use the control signal in order to simultaneously record marks on the tape, which are also referred to as indices. The beginning of a sequence marked by the index may then be searched for, for example, in a fast search mode.

FIG. 1 shows an exemplary rectangular control signal C recorded on a tape and a waveform H output by a read head reading the signal C.

The control signal C, as shown in FIG. 1, is cyclical. As is shown in FIG. 1, the first two periods of the signal have a 50%-duty cycle corresponding to a situation where the tape is not marked with an index. For the third period, signal C has a 27.5%-duty cycle, and for the fourth period a 60%-duty cycle. The 27.5% and 60% duty cycles are standardized ratios corresponding to binary values "1" and "0", respectively. Thus, in order to mark the tape with a digital index of n-bits, a combination of 27.5% and 60% duty cycle signals is recorded for n successive periods of signal C.

The control head behaves like a derivator, deriving the control signal C. Thus, as shown, signal H provided by the control head comprises a positive pulse at each rising edge of signal C and a negative pulse at each falling edge of signal C. The rising edges of signal C are separated by a constant length period. By detecting the positive pulses of signal H (when a positive threshold Vs is exceeded), the tape's playing time can be counted in real time. In addition, the falling edges of signal C are modulated with respect to the rising edge and correspond to the above-mentioned duty cycles. Thus, by detecting the negative pulses of signal H (when a negative threshold -Vs is exceeded), it is possible to check whether the control signal C includes binary data 0, 1, or not.

The marking of an index on the tape when, for example, a user desires to spot a sequence on the tape, is made as follows. A marking circuit, operating in a read mode, detects a rising edge of signal C, switches to a write mode, writes a high value during the required time interval (corresponding to 27.5% or 60% of the period), and then writes a low value, at least during a time interval corresponding to 60% of the period in case a high value was previously written during 27.5% of the period. Then, the marking circuit must rapidly switch again to the read mode in order to read the next rising edge of signal C. The transition from the write mode to the read mode raises problems for demagnetizing the read head and stabilizing a read amplifying circuit so that parasitic pulses are not recorded on the tape and so that the next rising edge of the control signal is correctly detected. These problems are described in more detail with reference to FIG. 2.

FIG. 2 is a schematic diagram of a conventional control circuit including the read head 10. A terminal A of the read head 10 is connected to a non-inverting input V+ of a high gain differential amplifier 12, and a second terminal B of the read head 10 is connected to the inverting input V- of amplifier 12. Input V+ of amplifier 12 is biased by a resistor R1 connected to a reference voltage Vref which is set to be approximately equal to half a supply voltage VCC. Input V- of amplifier 12 is further connected to ground through a capacitor C1 and to an output H of the amplifier 12 through a resistor R2. The specific connections of amplifier 12 regulate a quiescent voltage output by the amplifier to a value approximating Vref, independent of an offset voltage input to the amplifier. A time constant R2×C1 of the resistor, capacitor combination is selected so as to obtain, across capacitor C1, a constant voltage substantially equal to the mean output voltage H of amplifier 12.

Output H of amplifier 12 is connected to a hysteresis comparator 14 having a high threshold value Vs and a low threshold value -Vs, as shown in FIG. 1. The comparator 14 detects the positive and negative voltage pulses of signal H. If the quiescent voltage of amplifier 12 was not regulated as described above, it could vary as a function of the input offset voltage which, in turn, significantly varies as a function of temperature. Without such regulation, the comparator 14 might not be able to detect the positive or negative pulses of signal H. The output of the comparator 14 is used by counting circuits (not shown) for counting, in real time, the unwinding of the tape and for detecting the indices recorded on the tape.

Terminal B of the read head 10 is also connected to voltage Vref through a switch S1. Terminal A is also connected to voltage VCC through a switch S2 and to a current source I1 through a switch S3. Additionally, terminal A is connected to a switch S4 connected in series to an adjustable current source I2. The adjustable current source I2 is controlled by an inverting amplifier 16, an input of the inverting amplifier being connected to the voltage supply VCC through a current source I3 and to ground through a capacitor C2. Capacitor C2 can be shorted to ground by a switch S5.

Resistor R2 can be shorted through a switch S6.

The output of comparator 14 can be set to a high impedance through a switch S7.

Switches S1-S7 are controlled by a control circuit (CTRL) 18, in a way described hereinafter, to set the head to read or write mode and, in the write mode, to record on the tape a binary value "0" during the activation of a signal B0, or to record a binary value "1" during the activation of a signal B1. The sequencing of switches S1-S7 is determined by the control circuit 18 as a function of a voltage across capacitor C2, wherein the capacitor C2 together with source I3 and switch S5 comprises a voltage ramp generator.

FIG. 3 shows the waveforms of various signals that appear in the circuit of FIG. 2 when it is desired to write a binary "0" value on the tape. The given example corresponds to the writing of a square wave having a 60% duty cycle, that is, to the writing of a binary value "0".

Referring to FIG. 3, curve B0 represents the wave-form of signal B0 for the writing of a binary "0". Curve C represents the waveform of the control signal to be recorded on the tape. Curves S1-S7 represent the control signals to be applied to switches S1-S7, respectively, a high level of the signals corresponding to a switched-on condition. Curve VC2 represents the voltage across the ramp generator capacitor C2. Curve VH represents, in solid lines, the voltage signal V+ at the non-inverting input of amplifier 12 (terminal A), and, in dashed lines, the voltage signal V- at the inverting input of amplifier 12 (terminal B). The voltage V- also represents the voltage signal across the capacitor C1. Curve I₁₀ represents the current in the read head 10. Curve H represents the output voltage signal of amplifier 12.

The time interval during which signal C is at a high level (corresponding to 27.5% or 60% of the period of signal C) is not to scale, as shown in FIG. 3, in order to better show the major events occurring for the last 40% of the period of signal C.

At a time t0, an order to write a "0" is provided to the tape in the form of a pulse B0. At t0, the control circuit is in the read mode (signals S1-S6 are at a low level and signal S7 at a high level) and remains in the read mode until the occurrence of the next rising edge of the control signal C, which occurs at a time t1.

At a time t2, prior to the possible occurrence of a falling edge of the control signal C (corresponding to 27.5% of the period of signal C after time t1 for a recorded binary value 1), the write mode of the tape starts operating. Simultaneously, switch S7 is turned off so that the variations of the output signal H of amplifier 12 are not taken into account during this write mode and during a subsequent transition mode to be discussed below. The write mode comprises two phases, labeled as 1 and 2 in curve I₁₀.

Phase 1 includes allowing a positive current to flow through head 10 so as to write a high level signal on the tape. To achieve this function, switch S1 is turned on in order to connect terminal B of the read head 10 to the voltage Vref; switch S2 is turned on in order to connect terminal A of the read head 10 to the voltage supply VCC. Thus, a voltage VCC-Vref is applied across head 10. Switch S1 is turned on in order to maintain a constant voltage across capacitor C1 so that the voltage is not affected by any subsequent variations of the output voltage H of amplifier 12. Before switch S1 is biased on, voltage V- across capacitor C1 is, as shown by curve V_(H), slightly higher than voltage Vref. The voltage applied to the read head 10 during this phase generates a positive current in the read head 10 which writes on the tape a high level value of the control signal. Phase 1 lasts up to a time t3 occurring, in the present example, at a time corresponding to 60% of the period of signal C. In the alternative, for the write operation of a binary value 1 on the tape (not shown), time t3 occurs at a time corresponding to 27.5% of the period of signal C.

At a time t3, phase 2 of the write mode begins. Phase 2 consists in writing a falling edge of the control signal C. In all cases (writing of a binary value 0 or 1 on the tape), phase 2 may be slightly prolonged beyond 60% of the period of signal C, for example, up to 61%. Alternatively, during the writing operation of a binary value 1 (not shown), phase 2 lasts from 27.5% to 60% of the period of signal C in order to erase a possible binary value 0 that would have been previously recorded. Switch S2 is turned off and switch S3 is turned on. Switch S3 connects terminal A of the read head 10 to ground through the current source I1. Thus, a negative current is imposed on the read head 10 that writes a binary "0" value on the tape. Voltage V+ drops to a low value corresponding to the flow of a negative current through the head.

At a time t4, the write mode stops and a transition mode starts in order to rapidly switch from the write mode to the read mode before the occurrence of the next rising edge of signal C. The transition mode comprises three successive phases, labeled as 3 to 5 in curve I₁₀. Phase 3 is an undesired latency phase. After the write mode, a current in the inductive component of head 10 must be restored sufficiently slowly to a zero value so that no overvoltage is generated and no parasitic pulse is written on the tape. To achieve this function, it is desirable to impose to the read head 10 a current ramp having a decreasing absolute value. At time t4, switch S5 is turned off, thereby allowing capacitor C2 to be charged by the current source I3. Voltage VC2 across capacitor C2 thus starts increasing linearly. At time t4, switch S3 is also turned off, and switch S4, which connects the variable current source I2 to terminal A of the head 10, is turned on. Since the variable current source I2 is negatively controlled by voltage VC2, the current I2 decreases in absolute value as a function of increasing voltage VC2. Current I2 decreases from a maximum theoretical value I2max selected as close as possible to value I1 of the current present in head 10 at the end of phase 2. However, the value I2max must be chosen higher than I1 in order to prevent, due to manufacturing differences and temperature variations, the current I2max from becoming lower than I1, which would generate a current step in the head 10 and result in a writing of a parasitic pulse on the tape. Thus, at time t4, and until current I2 reaches value I1, source I2 is saturated, that is, the head 10 consumes a current lower than the current that source I2 tends to provide. Since the slope of current 12 is low, phase 3 is long-lasting.

At a time t5, the current of source I2 reaches value I1, and phase 4 provided for effectively discharging the current accumulated in the head 10 starts.

At a time t6, the current in head 10 is cancelled. Time t6 is approximately detected when voltage VC2 reaches an adequately selected threshold value V1. At time t6, the voltage signal V+ is beyond its initial value and voltage V- (that is, the voltage across capacitor C1) is below its initial value. The function of phase 5 is to restore as fast as possible the input voltages V+, V- of amplifier 12 to their initial values. In phase 5, switches S1 and S4 are off. Amplifier 12 thus returns to its initial configuration for regulating the quiescent voltage. Additionally, switch S6 that shorts resistor R2 is turned on. Thus, an equilibrium state of amplifier 12 is very rapidly reached due to the fast charging of the capacitor C1. As is shown in FIG. 3, the voltage signals V+ and V- rapidly recover their initial values.

Phase 5 must be sufficiently long so that voltages V+ and V- recover their initial values before the end of this phase. Phase 5 ends at a time t7 determined by the fact that the voltage VC2 across capacitor C2, that is still charged by source I3, reaches an adequately selected threshold value V2. Switch S6 is turned off and amplifier 12 is restored to its initial configuration operable for the slow regulation of the quiescent voltage. The read mode could be restored at time t7. In fact, it is restored at a subsequent time t8 when switch S7 is turned on again in order to enable the output of comparator 14.

A problem with the above described operation is that, due to the abrupt switching off of switch S6, a biasing current i of amplifier 12 has sufficient time for charging capacitor C1 before the regulation of the quiescent voltage begins. As represented by curve VH, voltage V- increases slightly, then starts decreasing slowly due to the quiescent voltage regulation.

As represented by curve H, the output of amplifier 12 is the sum of the voltage signals (V+)-(V-). The output of the amplifier is not accounted for between time periods t2 and t8 due to the fact that switch S7 remains off between these time intervals. Time t8 is determined by the fact that voltage VC2 across capacitor C2 reaches a threshold value V3 slightly lower than the end value reached by voltage VC2 (approximately equal to VCC).

At a time t9, the next rising edge of signal C occurs. As represented by curve H, the output voltage of amplifier 12 is, at time t9, still below its quiescent value, due to the parasitic charging of capacitor C2 by current i. If at the time t9 the output voltage of amplifier 12 is still too far from its quiescent value, the pulse corresponding to the rising edge of signal C may not be detected by amplifier 14. In practice, less than 40% of the period of signal C, i.e., approximately 15 milliseconds, is available to stabilize the output of amplifier 12 to an adequate value (allowing for the detection of the next rising edge of signal C) after a write operation.

However, the present invention is directed to lessening the time required to stabilize the read head after a write mode, which becomes difficult with the conventional above-described circuit.

SUMMARY OF THE INVENTION

An object of the invention is to provide a switching circuit for rapidly switching between a read and a write mode of a read head.

The invention is directed to an apparatus for accelerating a decrease of an initial current output by a variable current connected to a first terminal of said read head source during a latency phase, and for providing progressive control of a switch shorting out a resistor between an input of a differential amplifier and an output of the amplifier.

More particularly, the invention provides a device for switching from the write mode to the read mode wherein the read head is connected between a first and a second input terminal of the differential amplifier, the second input terminal being connected to a constant voltage through a capacitor and to the amplifier output through the resistor. The apparatus includes a circuit for generating a voltage ramp, after a write operation; a circuit for discharging an initial current in the read head proportionally to the voltage ramp between the beginning of the ramp and a first threshold voltage of the ramp; a switch for shorting the resistor to a low value, at least between the first threshold voltage and a second ramp threshold voltage; a detector for detecting the second threshold voltage and for providing, in the vicinity of the threshold, a progressive control of the switching means.

According to an embodiment of the invention, the switch includes two switching transistors of the same type disposed in parallel with the resistor and arranged such that dissimilar terminals of the transistors are connected together.

According to an embodiment of the invention, the detector includes two transistors connected as a differential amplifier stage, a first input of which receives a voltage equal to the second voltage threshold value and a second input which receives the voltage ramp, and a current mirror for supplying to a base of each switching transistor a current proportional to the current flowing in either one of the differential stage transistors.

According to an embodiment of the invention, the apparatus also includes a current mirror for duplicating the current and presenting to the bases of the switching transistors the current flowing in either one of the differential stage transistors.

The apparatus further includes a variable current source for discharging the current of the read head, the decrease of the current source being controlled proportionally to the voltage ramp between the beginning of the ramp and the first threshold value of the ramp, the current source having an initial absolute maximum value higher than the initial current of the read head. According to an embodiment of the invention, a circuit is provided for accentuating the ramp slope between the beginning of the ramp and a time at which the discharge current becomes equal to the current present in the read head.

The ramp generator circuit includes a second capacitor charged through a first constant current source. According to an embodiment of the invention, the voltage ramp accentuator circuit includes a second constant current source having a higher value than that of the first constant current source, and a switch for connecting the second source in parallel with the first source between the beginning of the voltage lamp and the time at which the current of the variable current source becomes equal to the current present in the read head.

According to an embodiment of the invention, the voltage ramp accentuator circuit also includes a circuit for detecting the saturation state of the variable current source, and for controlling the switch when the variable current source is saturated.

According to an embodiment of the invention, the voltage ramp accentuator circuit further includes a detection transistor serially connected between the variable current source and an input of a current mirror, an emitter of said transistor being connected to the variable current source output, a current mirror output feeding the second capacitor, a base of the transistor receiving a voltage at least equal to the sum of the saturation voltage of the variable current source and the voltage drop between the base and emitter of the transistor.

The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a signal diagram illustrating signals according to a prior art circuit;

FIG. 2 is a schematic diagram of a prior art control circuit;

FIG. 3 is a more detailed signal diagram of the prior art circuit;

FIG. 4 is a schematic diagram of an embodiment according to the present invention;

FIG. 5 represents in more detail an embodiment of a detector of the circuit of FIG. 4; and

FIG. 6 is a signal diagram of the various signals of the embodiment of the circuit of FIG. 4 during the transition from the write mode to the read mode.

DETAILED DESCRIPTION

In FIG. 4, the same components as are shown in FIG. 2 are designated with the same reference numbers. In order to simplify the drawing, only some of the components of FIG. 2 are shown again.

According to the present invention, an inverting input of a comparator 20 is connected between a switch S4 and a controlled current source I2. A non-inverting input of the comparator 20 is connected to a voltage (Vsat) substantially equal to the saturation voltage of the current source I2. The output of comparator 20 is connected to and controls a switch S8 disposed in series with a current source I4. Source I4 and switch S8 are disposed in parallel with a current source I3 connected in series to charging capacitor C2. The absolute value of the current provided by the current source I4 is larger than the current provided by the source I3.

According to the preferred embodiment of FIG. 4, as described with reference to FIG. 6, a latency phase between a write mode and a read mode of the read head is reduced to a negligible duration.

According to another aspect of the present invention, a differential amplifier stage including two NPN transistors Q1 and Q2, compares a voltage across capacitor C2 with a threshold voltage V2 that can be set to have a value identical to the voltage shown in FIG. 6. transistors Q1 and Q2 are connected through emitter resistors R3 and R4 to a current source I5, which is connected to ground. Source I5 can be enabled or disabled by a signal E5. Resistors R3 and R4 are useful for limiting and linearizing the gain of the differential amplifier stage. The threshold voltage V2 is applied to a base of transistor Q1, and the voltage VC2 across capacitor C2 is applied to the base of transistor Q2. A collector of transistor Q2 is connected to a supply voltage VCC and a collector of transistor Q1 is connected to the input of a current mirror M1, which is in turn connected to voltage VCC. The current mirror M1 includes two outputs feeding a base of two NPN transistors Q3 and Q4 disposed in parallel, and arranged such that dissimilar terminals of the transistors are connected. Transistors Q3 and Q4 comprise element S6 for shorting out the resistor R2.

As will be seen with reference to FIG. 6, this circuit provides a mechanism for substantially cancelling any variation of a voltage signal V- at the end of phase 5 of the prior art control circuit.

FIG. 5 is a schematic diagram of an embodiment of a detector assembly comprising the comparator 20, the current source I4 and a detector switch S8. Capacitor C2 is charged by the current source I3 and, if required, also by an output of a current mirror M2 connected to the supply voltage VCC. An NPN transistor Q5 is disposed in series between the controlled current source I2 and an input of the current mirror M2. Transistor Q5 receives at its base a signal which is the sum of the above-mentioned voltage Vsat and of a voltage Vbe (base-emitter voltage of transistor Q5). With this arrangement, when the voltage across the current source I2 is lower than Vsat, that is, when source I2 is saturated, transistor Q5 is conductive and the difference between the current provided by the current source I2 and current I₁₀ is provided to the input of current mirror M2. This input current can be multiplied by a factor K in the current mirror M2 and provided to capacitor C2. When the current source I2 is not saturated, transistor Q5 does not conduct and capacitor C2 is charged by the current source I3 only.

FIG. 6 shows the waveforms of the various signals of the circuit of FIG. 4 in a situation identical to that of FIG. 3. FIG. 6 shows again the same curves, times and phases, that are designated with the same references as in FIG. 3.

The writing of a binary "1" or a "0" value on the tape, occurring between times t0 and t4, is made as described with reference to the prior art circuit of FIG. 3. Hence, phase 1 of the present invention for writing a high value and phase 2 for writing a low value on the tape, are identical to that of the prior art circuit.

At time t4, it is desired to charge capacitor C2 in order to generate a voltage ramp and to progressively discharge an initial current in the read head according to a current output by the controlled current source I2. Switch S5 is turned off and switch S4 is turned on. The current source I2 is initially saturated because it is designed to have an initial absolute value I2max, higher than the initial value I1 of the current present in the read head 10 at time t4. The saturation state of current source I2 is detected by comparator 20 that turns switch S8 on, thus setting in parallel the high value current source I4 with the current source I3. Thus, the voltage across capacitor C2 very rapidly charges. Simultaneously, the current theoretically provided by source I2 decreases so as to rapidly reach the current value I1 at time t5. The source I2 is then no longer saturated, which is detected by the comparator 20 which then turns switch S8 off. The capacitor C2 is then charged solely by the current source I3 during a phase 4, an operation of the present invention is identical to the prior art circuit.

Thus, the present invention significantly decreases the length of the latency phase 3 of the prior art embodiment.

At a time t6, the current source I5 of the differential amplifier stage Q1, Q2 is enabled. Stage Q1, Q2 is then saturated because the base voltage V2 of transistor Q1 is sufficiently higher than the base voltage VC2 of transistor Q2. Transistor Q1 derives the whole current of source I5. The current of transistor Q1 is duplicated and fed to the bases of transistors Q3 and Q4 comprising control element S6. One of the transistors Q3, Q4, will as a function of the polarities of the voltages signals V- and H, saturate and short out resistor R2. An initial stabilization phase 5, identical to phase 5 of FIG. 3, is obtained.

When the voltage VC2 approaches the threshold voltage V2, the differential amplifier stage Q1, Q2 enters a linear mode. As voltage VC2 continues increasing, the current in transistor Q1 decreases whereas the current in transistor Q2 increases. Thus, the current provided to the bases of transistors Q3, Q4 of control element S6 decreases, and transistors Q3, Q4 are progressively pinched off. Transistors Q3, Q4 are completely biased off when the voltage VC2 reaches a value sufficiently high with respect to V2. Then, the differential stage Q1, Q2 is saturated and the transistor Q2 derives the whole current of source I5.

Thus, resistor R2 is progressively switched from a short circuit to its actual resistance value, which leaves time for the regulation loop of the quiescent voltage to compensate for the charging of capacitor C1 by the polarization current i. As shown in FIG. 6, about time t7 of the progressive switching, there is practically no variation of the voltage signal V-. Thus, the next rising edge of signal C can occur any time after time t7. Thus, the combination of both the circuit for decreasing the latency phase 3 and the circuit for progressively switching the resistor R2 provides a significant decrease of the required time for switching head 10 from the write mode to the read mode.

As was the case of the prior art circuit of FIG. 3, the output of amplifier 72 is enabled when voltage VC2 reaches the threshold value V3. The control signal E5 of source I5, as above indicated, is enabled at time t6, and can be disabled when transistor Q2 derives the whole current of source I5, for example, at time t8 when the output of comparator 14 is enabled.

Signals S1-S5 and S7 are conventionally provided by the control circuit 18 of FIG. 2. Those skilled in the art will be capable of fabricating a circuit providing a suitable enabling signal E5, for example with a switch disposed in series with source I5 and controlled through a window comparator having threshold values V1 and V3. Source I5 could be permanently enabled, which would cause control element S6 to be on from time t2. This would impair the output H of amplifier 12, which is unimportant since this output is not taken into account between times t2 and t8.

As is apparent to those skilled in the art, various modifications can be made to the above disclosed preferred embodiments. More particularly, for example, with respect to fabrication of the current sources and switches associated.

Having thus described one particular embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto. 

What is claimed is:
 1. An apparatus for switching a read head from a write mode to a read mode, the apparatus comprising:a read head having two ports, a first port connected to a first input terminal of a differential amplifier, a second port connected to a second input terminal of the differential amplifier, wherein the second port is linked to a constant voltage through a capacitor and to an output of the amplifier through a resistor; means for generating and outputting a voltage ramp, after a write operation of the read head; means for discharging an initial current in the read head, after the write operation, and a means for controlling said discharging means proportionally to said voltage ramp between a first time frame corresponding to a beginning voltage value of the voltage ramp and a second time frame corresponding to a first threshold voltage value of the voltage ramp; switching means for shorting out said resistor at least between the first time frame and a third time frame corresponding to a second threshold voltage value of the voltage ramp; and means for detecting the second threshold voltage and for progressively controlling, upon detection of said threshold voltage, the switching means.
 2. The apparatus of claim 1 wherein said switching means includes first and second switching transistors disposed in parallel with said resistor and connected such that an emitter of the first and second transistor is connected to a collector of the second and first transistor.
 3. The apparatus of claim 2, wherein said means for detecting the second threshold voltage and for progressively controlling said switching means includes:two transistors arranged as a differential amplifier, wherein a first input of said differential amplifier is connected to a voltage equal to said second threshold voltage and a second input of said differential amplifier is connected to an output of the means for generating for the voltage ramp; and means for supplying a current to a base of each said switching transistors proportional to the current flowing in either one of said differential amplifier transistors.
 4. The apparatus of claim 3, wherein the means for supplying a current to the transistor bases is a current mirror which duplicates the current flowing in either one of the differential amplifier transistors.
 5. The apparatus of claim 1, wherein:the means for discharging is a variable current source, said current source having an initial maximum current value larger than the initial current in said read head; and means for accentuating the voltage ramp slope between the first time frame and a fourth time frame corresponding to a point where the current output by the variable current source substantially equals the initial current in the read head.
 6. The apparatus of claim 5, wherein:said voltage ramp generating means includes a second capacitor charged through a first constant current source; and wherein said means for accentuating the voltage ramp slope includes a second constant current source having a larger output current value than that of said first constant current source, and a switching means for coupling the second constant current source in parallel with the first constant current source between the first time frame and the third time frame.
 7. The apparatus of claim 6, wherein said means for accentuating the voltage ramp slope includes means for detecting a saturation state of said variable current source, an output of said means for detecting connected to a control port of said switching means for controlling the switching means.
 8. The apparatus of claim 6, wherein said means for increasing the voltage ramp slope further includes a detection transistor serially connected between an output of said variable current source and an input of a current mirror and wherein an emitter of the transistor is connected to the variable current source input, an output of the current mirror is connected to said second capacitor, and a base of the transistor is connected to a voltage at least equal to the sum of the saturation voltage of said variable current source and a voltage drop between the base and emitter of the transistor.
 9. A method for decreasing a transition time of a read head from a write mode to a read mode, comprising the steps of:generating and outputting a voltage ramp, after a write operation of the read head; coupling a variable current source to a first terminal of said read head after said write operation; detecting a saturation state of the variable current source; increasing a slope of the voltage ramp in response to said saturation detection; detecting when a current output by the variable current source substantially equals an initial current value in said read head immediately after said write operation; and reducing the slope of the voltage ramp upon said detection of the current output by said variable current source being substantially equal to said initial current value in the read head.
 10. The method of claim 9 wherein the step of coupling the variable current source to the first terminal of said read head further comprises the steps of:setting the value of the current output by the variable current source to have an initial absolute value larger than the initial current value in the read head.
 11. The method of claim 9 wherein a first and a second terminal of said read head is connected to a first and second input terminal of a differential amplifier and wherein the second input terminal is connected to a constant voltage through a capacitor and to an output of the differential amplifier through a resistor, further comprising the steps of:continuously shorting out said resistor between a first time frame corresponding to a beginning voltage value of said voltage ramp and a second time frame corresponding to a threshold value output of said voltage ramp; detecting said threshold value of said voltage ramp; and continuously eliminating the short circuit across said resistor after detecting said threshold value.
 12. An apparatus for switching a read head from a write mode to a read mode, the apparatus comprising:the read head having two ports, a first port being connected to first input terminal of a differential amplifier and a second port being connected to a second input terminal of the differential amplifier, to a constant voltage through a capacitor and to an output of the amplifier through a resistor; a circuit generating and outputting a voltage ramp, after a write operation of the read head; a discharging circuit connected to the first port of the read head and outputting a variable current discharging an initial current in the read head, after the write operation; a control circuit including an input coupled to an output of the voltage ramp generating circuit which varies the current output by the discharging circuit proportionally to said voltage ramp between a first time frame corresponding to a beginning voltage value of the voltage ramp and a second time frame corresponding to a first threshold voltage value of the voltage ramp; a switch connected in parallel with and shorting out said resistor at least between the first time frame and a third time frame corresponding to a second threshold voltage value of the voltage ramp; and a detection and control circuit including an input connected to the output of the voltage ramp circuit which upon detection of the second threshold voltage progressively controls the switch.
 13. The apparatus of claim 12 wherein said switch includes a first and second switching transistor disposed in parallel with said resistor and connected such that an emitter of the first and second transistor is connected to a collector of the second and first transistor.
 14. The apparatus of claim 13, wherein the detection and control circuit includes:first and second transistors arranged as a differential amplifier, wherein a base of said first transistor is connected to a voltage equal to said second threshold voltage and a base of said second transistor is connected to the output of the circuit generating the voltage ramp; and a circuit supplying a current to a base of each said first and second transistors proportional to the current flowing in either one of said differential amplifier transistors.
 15. The apparatus of claim 14, wherein the circuit supplying the current to the transistor bases is a current mirror which includes an input connected to a collector of said first transistor which duplicates the current flowing in either one of the differential amplifier transistors.
 16. The apparatus of claim 12, wherein:the discharging circuit is a variable current source, said current source having an initial absolute current value larger than the initial current in said read head; and the apparatus further comprises a circuit increasing the voltage ramp slope between the first time frame and a fourth time frame corresponding to a point where the current output by the variable current source substantially equals the initial current in the read head.
 17. The apparatus of claim 16, wherein:said voltage ramp generating circuit includes a second capacitor charged through a first constant current source; and wherein said circuit increasing the voltage ramp slope includes a second constant current source, having a larger output current value than that of said first constant current source and a switch coupling the second constant current source in parallel with the first constant current source, between the first time frame and the third time frame.
 18. The apparatus of claim 17, wherein said circuit increasing the voltage ramp slope includes a detection and control circuit including an input connected to the output of the variable current source which detects a saturation state of said variable current source, and an output of said detection and control circuit is connected to a control port of said switch for controlling the switch.
 19. The apparatus of claim 17, wherein said circuit increasing the voltage ramp slope further includes a detection transistor serially connected between an output of said variable current source and an input of a current mirror and wherein an emitter of the transistor is connected to the variable current source output, an output of the current mirror is connected to said second capacitor, and a base of the transistor is connected to a voltage at least equal to the sum of the saturation voltage of said variable current source and a voltage drop between the base and emitter of the transistor. 